The Experimental Forecast Program

Tag: soundings

It is remarkably difficult to predict convection initiation. It appears we can predict, most times (see yesterdays post for a failure), the area under consideration. We have attempted to pick the time period, in 3 hour windows, and have been met with some interesting successes and failures. Today had 2 such examples.

We predicted a time window from 16-19 UTC along the North Carolina/South Carolina/ Tennessee area for terrain induced convection and along the sea breeze front. The terrain induced storms went up around 18 UTC, nearly 2 hours after the model was generating storms. The sea breeze did not initiate storms, but further inland in central South Carolina there was one lone storm.

The other area was in South Dakota/North Dakota/Nebraska for storms long the cold front and dryline. We picked a window between 21-00 UTC. It appears storms initiated right around 00 UTC in South Dakota but little activity in North Dakota as the dryline surged into our risk area. Again the suite of models had suggested quick initiation starting in the 21-22 UTC time frame, including the update models.

In both cases we could isolate the areas reasonably well. We even understood the mechanisms by which convection would initiate, including the dryline, the transition zone, and the where the edge of the deeper moisture resided in the Dakotas. For the Carolinas we knew the terrain would be a favored location for elevated heating in the moist air mass along a weak, old frontal zone. We knew the sea breeze could be weak in terms of convergence, and we knew that only a few storms would potentially develop. What we could not adequately do, was predict the timing of the lid removal associated with the forcing mechanisms.

It is often observed in soundings that the lid is removed via surface heating and moistening, via cooling aloft, or both processes. It is also reasonable to suspect that low level lifting could be aiding in cooling aloft (as opposed to cold advection). Without observations along such boundaries it is difficult to know what exactly is happening along them, or even to infer that our models correctly depict the process by which the lid is overcome. We have been looking at the ensemble of physics members which vary the boundary layer scheme, but today was the first day we attempted to use them in the forecast process.

It was successful in terms of incorporating them, but as far as achieving understanding, that will have to come later. It is clear that understanding the various structures we see, and relating them to the times of storm initiations will be a worthwhile effort. Whether this will be helpful to forecasting, even in hindsight, is still unknown.

Every Monday we get a new group of participants and this week we spent time discussing all the issues from their perspective. From an aviation, airport, and airplane perspective to towering cumulus from the mountains. We then discussed the forecast implications from verification, to model data mining, to practical use of forecast data and again learned the lesson that forecasters already know: you only have so much time before your forecast is due and it better contain the answer!

Our forecast has evolved into a 3 hour categorical product of convection initiation where we determine the domain and the time window. We then go a step further and forecast individually a location where we think the first storm will be in our domain during our time period. We then forecast the time we think it will occur and our uncertainty. Then we assign our confidence that CI will occur within 25 miles of our point. It might sound easy, but it takes some serious practice to spin up 10 people on observations, current and experimental guidance, AND lots of discussion about the scenario or scenarios at play. We have a pretty easy going group, but we do engage in negotiations about where to draw our consensus categorical risk and over what time period we are choosing. It is a great experience to hear everyone’s interpretation of the uncertainty on any number of factors at play.

Monday was a great practice day with a hurried morning forecast, and terrain induced CI forecast in the afternoon. Tuesday we were off to a good start with all products nominal, 10+ forecasters, and action back out on the Plains with plenty of uncertainty. The highlights from today included the introduction of ensemble soundings via the BUFKIT display (Thanks Patrick Marsh!). This garnered a lot of attention and will be yet another new, exciting, and valuable visualization tool. The aviation forecasters shared tremendous insights about their experiences and even showed a movie of what they face as airplane traffic gets shuffled around thunderstorms. It was a glimpse of exactly the sorts of problems we hope to address with these experimental models.

These problems are all associated with the CI problem, on every scale (cloud, storm, squall line scales). The movie highlighted the issue of where and when new convection would fill in the gaps, or simple occupy more available air space, or block and airport arrival, or when convection would begin to fizzle. Addressing these issues is part of the challenge and developing guidance relies almost exclusively on how we define convection initiation in the models and observations. We have some great guidance and it is clear that as we address more of the challenges of generic CI we will require even more guidance to account for the sheer number of possibilities of where (dx, dy and dz), when, how, and if CI occurs.

As an example, we issued our first night time elevated convection forecast. As it turns out, we could be verifying this by observing rain in OK tonight. Our experimental guidance was inadequate as we have very little data aloft except soundings from the fine resolution models. So we looked at more regular models while using what was available from the fine resolution models, like reflectivity and CI points. This highlights a unique operational challenge that we all face: Data overload and time intensive information extraction. The forecast verification for tonight should be quite revealing and should provide more insight than I am prepared to discuss this evening.